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प्रश्न
विकल्प
40 cm
25 cm
`1/40` cm.
`1/25` cm
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उत्तर
The power of a convex lens is 2.5 dioptres. Its focal length is 40 cm.
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संबंधित प्रश्न
(a) Draw a ray diagram to show the formation of image by a concave lens when an object is placed in front of it.
(b) In the above diagram mark the object distance (u) and the image distance (v) with their proper signs (+ve or –ve as per the new Cartesian sign convention) and state how these distances are related to the focal length (f) of the concave lens in the case.
(c) Find the nature and power of a lens which forms a real and inverted image of magnification –1 at a distance of 40 cm from the optical centre.
A student places a candle flame at a distance of about 60 cm from a convex lens of focal length 10 cm and focuses the image of the flame on a screen. After that he gradually moves the flame towards the lens and each time focuses the image on the screen.
(a) In which direction-toward or away from the lens, does he move the screen to focus the image?
(b) How does the size of the image change?
(c) How does the intensity of the image change as the flame moves towards the lens?
(d) Approximately for what distance between the flame and the lens, the image formed on the screen is inverted and of the same size?
Where should an object be placed so that a real and inverted image of the same size as the object is obtained using a convex lens?
Draw a labelled ray diagram to show how a ray of light is refracted when it passes:
from an optically denser medium into air.
A student did an experiment with a convex lens. He put an object at different distances 25 cm, 30 cm, 40 cm, 60 cm and 120 cm from the lens. In each case he measured the distance of the image from the lens. His results were 100 cm, 24 cm, 60 cm, 30 cm and 40 cm, respectively. Unfortunately his results are written in wrong order.
What is the focal length of this lens?
An object 50 cm tall is placed on the principal axis of a convex lens. Its 20 cm tall image is formed on the screen placed at a distance of 10 cm from the lens. Calculate the focal length of the lens.
What kind of lens is used to correct
long-sightedness?
Define the term principal axis of a lens.
The given below figure shows an object OA and its image IB formed by a lens
. 
draw suitable rays to locate the lens and its focus.
A convex lens forms an image of an object equal to the size of the object. State two more characteristics of the image.
A lens forms an upright and magnified image of an object State whether the image is real or virtual
An object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. Find (i) the position (ii) the magnification and (iii) the nature of the image formed.
If an object is placed in front of a convex lens beyond 2F1, then what will be the position, relative size, and nature of an image which is formed? Explain with a ray diagram.
An object 4.0 cm in size, is placed 25.0 cm in front of a concave mirror of focal length 15.0 cm.
- At what distance from the mirror should a screen be placed in order to obtain a sharp image?
- Find the size of the image.
- Draw a ray diagram to show the formation of the image in this case.
State the position of object, position of image, nature of image when: Convex lens is used as objective lens of photographic camera.
State the nature and position of the object on the principal axis to obtain a real image of the same size
Why do we say that the ‘2F’ and ‘F’ points of a convex lens can be regarded as a sort of ‘turning points’ as far as the nature of the image formed by it is concerned?
A candle is placed between f and 2f a convex lens. Draw a ray diagram showing the position of the image.
: Object near the lens : : ______ : 
A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
